One of the most promising recent developments in cancer research is the use of immunotoxins for chemotherapy. An immunotoxin is a hybrid molecule, being a conjugate between a potent protein toxin and an antibody. Such agents have the potential of being both highly efficacious and specific. They have not, however, lived up to their potential as they have not been able to enter their target cells efficiently to kill them. The long-range goal of this research is to understand the mechanism of how an immunotoxin enters a cell. The information generated by this research may be used to design practical, efficacious chemotherapeutic agents. The mechanism of toxin entry will be studied by studying the interaction of immunotoxins made with diphtheria toxin with artificial lipid membranes. This will allow a direct measurement of those factors that control the transport of the toxin across a membrane, and it is just such a transport step that limits the entry of immunotoxins into a cell. The artificial lipid membranes that will be used as models are the planar lipid bilayer and the unilammellar lipid vesicle. These two systems allow the collection of complementary data: the planar bilayer allows a very sensitive measurement of the binding of the immunotoxins to the membrane and their insertion into the membrane and the lipid vesicles allow the measurement of the actual transport of the toxin across the membrane. In both systems, the conditions to which the immunotoxin and the membrane are exposed will be varied, so as to determine the optimum conditions for toxin transport. These results will then be correlated with the toxicity of the immunotoxins for their target cells. This correlation will allow a pinpointing of those conditions that affect the transport of the toxins across the cell membrane as well as the artificial lipid membrane, and so will give direct physiological relevance to the results. Thus, the specific aims are to measure the ability of the immunotoxins to insert into membranes and transport themselves; to modify conditions such as pH, membrane voltage, and membrane composition in order to optimize transport; and to correlate the conditions for optimal transport with cytotoxicity. (HI)